U.S. patent application number 15/564802 was filed with the patent office on 2018-04-26 for method of predicting the concentration of asphaltenes using a first precipitant and correlation back to an asphaltene concentration measurement using a second precipitant.
The applicant listed for this patent is SCHLUMBERGER TECHNOLOGY COPORATION. Invention is credited to Farshid Mostowfi, Vincent Joseph Sieben.
Application Number | 20180113108 15/564802 |
Document ID | / |
Family ID | 57072815 |
Filed Date | 2018-04-26 |
United States Patent
Application |
20180113108 |
Kind Code |
A1 |
Mostowfi; Farshid ; et
al. |
April 26, 2018 |
METHOD OF PREDICTING THE CONCENTRATION OF ASPHALTENES USING A FIRST
PRECIPITANT AND CORRELATION BACK TO AN ASPHALTENE CONCENTRATION
MEASUREMENT USING A SECOND PRECIPITANT
Abstract
A method for determining the asphaltene content of oil includes
obtaining an oil sample, determining an optical spectrum of the oil
sample and removing asphaltenes from the oil sample by
precipitating asphaltenes using a first alkane precipitant. The
method also includes determining an optical spectrum of maltenes of
the oil sample and subtracting the optical spectrum of the maltenes
of the oil sample from the optical spectrum of the oil sample to
yield an optical spectrum of asphaltenes of the oil sample. The
method further includes using the optical spectrum of asphaltenes
of the oil sample to determine asphaltene content of the oil sample
using a second alkane precipitant.
Inventors: |
Mostowfi; Farshid;
(Lexington, MA) ; Sieben; Vincent Joseph;
(Cambridge, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCHLUMBERGER TECHNOLOGY COPORATION |
Sugar Land |
TX |
US |
|
|
Family ID: |
57072815 |
Appl. No.: |
15/564802 |
Filed: |
April 7, 2015 |
PCT Filed: |
April 7, 2015 |
PCT NO: |
PCT/US2015/024615 |
371 Date: |
October 6, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 21/314 20130101;
G01N 33/2835 20130101; G01N 21/05 20130101; G01N 33/2823 20130101;
G01N 21/75 20130101; E21B 49/10 20130101; E21B 49/08 20130101; G01N
31/02 20130101; E21B 49/0875 20200501; G01N 21/84 20130101; G01N
2021/8472 20130101 |
International
Class: |
G01N 33/28 20060101
G01N033/28; G01N 31/02 20060101 G01N031/02; G01N 21/05 20060101
G01N021/05; G01N 21/31 20060101 G01N021/31; G01N 21/75 20060101
G01N021/75; G01N 21/84 20060101 G01N021/84; E21B 49/08 20060101
E21B049/08 |
Claims
1. A method for determining the asphaltene content of an oil,
comprising: obtaining an oil sample; determining an optical
spectrum of the oil sample; removing asphaltenes from the oil
sample to yield maltenes of the oil sample by precipitating
asphaltenes using a first alkane precipitant; determining an
optical spectrum of the maltenes of the oil sample precipitated
with the first alkane precipitant; subtracting the optical spectrum
of the maltenes of the oil sample from the optical spectrum of the
oil sample to yield an optical spectrum of asphaltenes of the oil
sample; and using the optical spectrum of asphaltenes of the oil
sample to determine asphaltene content of the oil sample using a
second alkane precipitant.
2. The method of claim 1, wherein the first alkane precipitant has
a higher order than heptane.
3. The method of claim 1, wherein the second alkane precipitant has
an order lower than that of the first alkane precipitant.
4. The method of claim 1, wherein the oil sample is diluted with a
solvent before its optical spectrum is determined and all
subsequent operations are conducted on an undiluted oil sample.
5. The method of claim 4, wherein the solvent is one of toluene,
xylene, or 1-methylnaphthalene.
6. The method of claim 1, wherein the optical spectrum of the oil
sample is determined by analyzing at least a portion of the oil
sample with a spectrometer.
7. The method of claim 6, wherein the portion of the oil sample is
analyzed with the spectrometer by disposing the portion of the oil
sample in an optical flow cell and analyzing the at least a portion
of the oil sample disposed in the optical flow cell with the
spectrometer.
8. The method of claim 1, wherein the asphaltenes are removed from
the oil sample by mixing the oil sample with the first alkane
precipitant and removing precipitated asphaltenes from the
oil-precipitant mixture.
9. The method of claim 8, wherein the oil sample is mixed with the
first alkane precipitant in a microfluidic mixer.
10. The method of claim 8, wherein the precipitated asphaltenes are
removed from the oil-precipitant mixture using a microfluidic
filter.
11. The method of claim 1, wherein the optical spectrum of the
maltenes of the oil sample is determined by analyzing at least a
portion of the maltenes of the oil sample with a spectrometer.
12. The method of claim 11, wherein the portion of the maltenes of
the oil sample is analyzed with the spectrometer by disposing the
portion of the maltenes of the oil sample in an optical flow cell
and analyzing the at least a portion of the maltenes of the oil
sample disposed in the optical flow cell with the spectrometer.
13. The method of claim 1, further comprising subtracting the
optical spectrum at a longer wavelength of the maltenes of the oil
sample from the optical spectrum at a shorter wavelength of the
maltenes of the oil sample and subtracting the optical spectrum at
a longer wavelength of the oil sample from the optical spectrum at
a shorter wavelength of the oil sample prior to subtracting the
optical spectrum of the maltenes of the oil sample from the optical
spectrum of the oil sample to yield the optical spectrum of the
asphaltenes of the oil sample.
14. The method of claim 13, wherein the longer wavelength of the
maltenes of the oil sample and of the oil sample is at about 800
nanometers and the shorter wavelength of the maltenes of the oil
sample and of the oil sample is at about 600 nanometers.
15. The method of claim 1, wherein the first alkane precipitant
includes one of octane, nonane, decane, undecane, dodecane,
tridecane, tetradecane, pentadecane, and hexadecane.
16. The method of claim 1, wherein the second alkane precipitant
includes one of heptane, hexane, and pentane.
17. The method of claim 1, wherein a correlation function relates
the optical spectrum of asphaltenes of the oil sample to the
asphaltene content of the oil sample using a second alkane
precipitant.
18. A method for determining the asphaltene content of oil,
comprising: obtaining an oil sample; determining an optical
spectrum of the oil sample; removing asphaltenes from the oil
sample to yield maltenes of the oil sample by precipitating
asphaltenes using a first alkane precipitant that has a higher
order than heptane; determining an optical spectrum of the maltenes
of the oil sample; subtracting the optical spectrum of the maltenes
of the oil sample from the optical spectrum of the oil sample to
yield an optical spectrum of asphaltenes of the oil sample; and
using the optical spectrum of asphaltenes of the oil sample
precipitated using the first alkane precipitant to determine an
optical spectrum of the asphaltenes of the oil sample precipitated
using a second alkane precipitant that has a higher order than
heptane; and using the optical spectrum of the asphaltenes of the
oil sample precipitated using the second alkane precipitant to
determine asphaltene content in the oil sample using a third alkane
precipitant having an order equal to or lower than heptane.
19. The method of claim 18, wherein a first correlation function
relates the optical spectrum of asphaltenes of the oil sample
precipitated using the first alkane precipitant to the optical
spectrum of the asphaltenes of the oil sample precipitated using
the second alkane precipitant.
20. The method of claim 19, wherein a second correlation function
relates the optical spectrum of the asphaltenes of the oil sample
precipitated using the second alkane precipitant to the asphaltene
content of the oil sample using the third alkane precipitant.
Description
BACKGROUND
Field
[0001] The present application relates to methods of predicting the
concentration of asphaltenes in a sample of oil, such as crude
oil.
Description of Related Art
[0002] Petroleum composition data plays a role in guiding both
upstream and downstream operations, including: predicting fluid
behavior inside a petroleum reservoir, providing flow assurance
during transportation of the petroleum, understanding potential
outcomes when mixing, blending, or diluting the petroleum, and
directing refinement processes. Separating the crude oil into its
constituent parts or "fractions" is a fundamental operation when
characterizing the composition of the crude oil.
[0003] Briefly, crude oils can be separated into two fractions:
asphaltenes and maltenes. The maltene fraction includes saturate,
aromatic, and resin molecules. Once separated, the fractions can be
quantified and analyzed. The asphaltenes are normally dissolved in
the crude oil, but can be precipitated out of solution by titrating
the crude oil with an alkane precipitant, such as heptane.
Thereafter, the precipitated asphaltenes can be filtered from the
resulting crude oil mixture, which constitutes the maltenes.
[0004] U.S. Pat. No. 8,269,961 (hereinafter "Mostowfi '961") and
International Patent Application Publication WO 2013/126732
(hereinafter "Mostowfi '732") describe how the optical spectrum of
the crude oil and the optical spectrum of the maltenes can be used
to determine the content of asphaltenes in the crude oil.
Specifically, Mostowfi '961 and Mostowfi '732 note that the optical
spectrum of crude oil is the sum of the optical spectra of its
constituent fractions, the asphaltenes and the maltenes. FIG. 1A
shows typical optical spectra of oil, asphaltenes, and maltenes in
the visible range. The optical spectrum of the maltenes can be
obtained after the asphaltenes are separated and the optical
spectrum of the crude oil can be obtained before the asphaltenes
are precipitated from the crude oil. The optical spectrum of
asphaltenes can be obtained by subtracting the optical spectrum of
maltenes from the optical spectrum of the crude oil.
[0005] In both Mostowfi '961 and Mostowfi '732 the determination of
asphaltene concentration is based on the difference between the
optical absorbance of crude oil before and after precipitation of
asphaltenes. Specifically, the difference in absorbance between the
crude oil with dissolved asphaltenes and the crude oil sample
without the precipitated asphaltenes (after titration with
heptane), is correlated to the weight concentration of asphaltenes
and such correlations correlate well to conventional wet chemistry
techniques such as those described in ASTM D6560, which are based
on titration of crude oil using a heptane precipitant. Thus, the
optical data is based on titration of the crude oil sample with the
same precipitant as used in the wet chemistry techniques used to
correlate the data. As a result, a direct correlation between the
optical data and the wet chemistry data is possible.
[0006] However, the maximum temperature at which the asphaltene
precipitant is effective in precipitating the asphaltenes is
limited by the boiling point of the precipitant (e.g.,
98.42.degree. C. for heptane at atmospheric pressure). For example,
rather than precipitating asphaltenes from the crude oil, at
temperatures above 98.42.degree. C. heptane would vaporize at
atmospheric pressure making optical absorbance measurements of the
crude oil and maltenes difficult. Thus, at elevated temperatures
above the boiling point of heptane, it would be technologically
challenging to use the technique described in Mostowfi '961 and
Mostowfi '732 based on heptane as the precipitant to correlate
optical data with wet chemistry data according to ASTM D6560.
SUMMARY
[0007] This summary is provided to introduce a selection of
concepts that are further described below in the detailed
description. This summary is not intended to identify key or
essential features of the claimed subject matter, nor is it
intended to be used as an aid in limiting the scope of the claimed
subject matter.
[0008] A method is described for predicting the concentration of
asphaltenes at elevated temperature conditions. In one embodiment,
the techniques described in Mostowfi '961 and Mostowfi '732 are
adapted by using a precipitant having a boiling point that is above
the temperature of the oil sample into which it is titrated.
However, using a precipitant that differs from what is specified in
ASTM D6560 results in a different partition of asphaltenes and
maltenes from the oil, such that optical data obtained from the
techniques described in Mostowfi '961 and Mostowfi '732 are not
directly correlatable to wet chemistry data obtained using ASTM
D6560. The method described herein includes a correlation of
obtained absorbance data of an oil sample with asphaltenes
precipitated by a high boiling point precipitant to a standard wet
chemistry measurement of asphaltenes precipitated at ambient
conditions using another precipitant, such as heptane in accordance
with ASTM D6560. Correlation back to the standard wet chemistry
measurement at ambient conditions according to ASTM D6560 is
helpful because it is a commonly accepted and performed testing
technique.
[0009] Also, another method is described for predicting the
concentration of asphaltenes by obtaining absorbance data from an
oil sample precipitated using a first alkane precipitant (e.g.,
dodecane) and correlating the obtained absorbance data to other
absorbance data corresponding to another oil sample precipitated
using a second alkane precipitant, such as decane. Where the
absorbance data corresponding to the second alkane precipitant is
itself correlated to asphaltene concentration data, the correlation
between the two sets of absorbance data can be used to correlate
the absorbance data corresponding to the first alkane precipitant
to the asphaltene concentration.
[0010] Illustrative embodiments of the present disclosure are
directed to a method and apparatus for determining the asphaltene
fraction of an oil sample. The method (and corresponding apparatus)
involves a sequence of operations including: obtaining an oil
sample; determining an optical spectrum of the oil sample; removing
asphaltenes from the oil sample by precipitating asphaltenes with a
first alkane precipitant; determining an optical spectrum of
maltenes of the oil sample precipitated with the first alkane
precipitant; subtracting the optical spectrum of the maltenes of
the oil sample from the optical spectrum of the oil sample to yield
an optical spectrum of asphaltenes of the oil sample; and using the
optical spectrum of the asphaltenes of the oil sample to determine
the asphaltene content of the oil sample using a second alkane
precipitant.
[0011] The first alkane precipitant may be an alkane having an
order higher than that of heptane, such as octane, nonane, decane,
undecane, dodecane, tridecane, tetradecane, pentadecane, or
hexadecane.
[0012] The second alkane precipitant may be an alkane having an
order lower than that of the first precipitant, such as heptane,
hexane, or pentane.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A is a graphical representation of the optical spectra
of an oil sample and its constituent maltenes and asphaltenes.
[0014] FIG. 1B is a flow chart representing an illustrative
embodiment of a method for determining the asphaltene content of
oil.
[0015] FIG. 2 is a graphical representation of optical absorbance
of various samples of crude oil titrated with various alkane
precipitants in which the optical spectra of the maltenes of the
sample have been subtracted from the optical spectra of the crude
oil samples.
[0016] FIG. 3A is a graphical representation of the absorbance data
from FIG. 2 plotted against asphaltene weight percentage determined
from wet chemistry testing performed for each crude oil sample.
Correlation functions are also illustrated in FIG. 3A indicating
that absorbance is a function of weight percentage and precipitant
used.
[0017] FIG. 3B is a plot of the slope of the correlation functions
plotted in FIG. 3A for each of the precipitants.
[0018] FIG. 4A is a graphical representation of the correlation
between the optical absorbance of asphaltenes precipitated using
decane and tetradecane.
[0019] FIG. 4B is a graphical representation of the correlation
between the optical absorbance of asphaltenes precipitated using
octane and tetradecane.
[0020] FIG. 5 is a stylized, graphical representation of an
illustrative embodiment of a system for determining the asphaltene
content of oil.
[0021] FIG. 6 is a schematic representation of a downhole tool that
performs the function of the system of FIG. 5.
DETAILED DESCRIPTION
[0022] Illustrative embodiments of the disclosed subject matter of
the application are described below. In the interest of clarity,
not all features of an actual implementation are described in this
specification. It will of course be appreciated that in the
development of any such actual embodiment numerous
implementation-specific decisions can be made to achieve the
developer's specific goals, such as compliance with system-related
and business-related constraints, which will vary from one
implementation to another. Moreover, it will be appreciated that
such a development effort might be complex and time-consuming but
would nevertheless be a routine undertaking for those of ordinary
skill in the art having the benefit of this disclosure.
[0023] As used herein, the term "microfluidics" or "microfluidic"
refers to a device, apparatus or system that deals with the
behavior, precise control, and manipulation of fluids that are
geometrically constrained to a small, commonly sub-millimeter,
scale. The device, apparatus, or system can employ small, commonly
sub-millimeter, scale channels that are etched into planar
substrates, such as glass, where networks of these embedded
channels transport the sample from one operation to the next. The
manipulation of small volumes of fluid enables precise control of
reagents and seamless automation of several consecutive
operations.
[0024] The present disclosure relates to a system and method for
determining the asphaltene content of an oil. Generally, oil is
made up of asphaltenes and maltenes. Separation of asphaltenes
requires addition of an alkane, such as heptane (C7), at, for
example, a 1:40 volume ratio, although higher volume ratios or
volume ratios as low as 1:1 or less are also satisfactory. Once the
precipitant (C7) is added, the asphaltenes aggregate and
precipitate out of the solution. The asphaltenes can then be
separated using a membrane filtration unit. The maltenes fraction
permeates through the filter and the optical spectrum of maltenes
may be measured downstream of the filter using a spectrometer.
[0025] The optical spectrum of the oil before asphaltene
precipitation can be measured and compared to the optical spectrum
of the maltenes after precipitation. In the case of the
microfluidic system disclosed in Mostowfi '732, a sample of the oil
before asphaltene precipitation is optionally diluted with toluene
at 1:80 volume ratio to reduce the darkness of the sample before
measurement of its optical spectrum. As noted in Mostowfi '961 and
Mostowfi '732, the difference between the optical spectrum of the
oil before asphaltene precipitation and the optical spectrum of the
maltenes after asphaltene precipitation can be correlated to the
mass concentration of the asphaltenes precipitated, e.g., with
heptane (C7), using conventional wet chemistry techniques such as
ASTM D6560. However, while the techniques described in Mostowfi
'961 and Mostowfi '732 work well under laboratory conditions using
heptane (C7) as the precipitant, heptane and toluene have
temperature limitations (high vapor pressure curves) that do not
readily permit their use with those techniques when performed under
elevated temperature conditions, and more specifically, at high
temperature conditions, such as those that may occur downhole,
where the presence of bubbles caused by solvent or precipitant
vaporization will make optical absorbance measurements
difficult.
[0026] To adapt the techniques described in Mostowfi '961 and
Mostowfi '732 for elevated temperature conditions, precipitants
(alkanes of higher order than that of heptane) that have boiling
points that are higher than heptane (C7) may be substituted for
heptane. The higher boiling point precipitants may have boiling
points that are higher than 200.degree. C., such as dodecane
(C.sub.12H.sub.26) or tetradecane
(CH.sub.3(CH.sub.2).sub.12CH.sub.3). However, using a precipitant
other than heptane results in a different partitioning of the
asphaltenes and maltenes from the oil sample, such that the
absorbance data obtained for asphaltenes precipitated with the
higher order alkane precipitant (i.e., the difference between the
optical spectrum of the oil and the optical spectrum of the
maltenes after the asphaltenes are precipitated with the higher
boiling point precipitant) may not be directly correlated to the
mass concentration of asphaltenes precipitated with heptane (C7) at
ambient conditions using conventional wet chemistry (e.g.,
gravimetric) techniques such as ASTM D6560.
[0027] To adjust for the differences in the different partitioning
of the oil fractions resulting from the use of higher order
precipitants, the absorbance data of the asphaltenes precipitated
with the higher order precipitant can be determined based on
asphaltene content data obtained from oil samples precipitated with
heptane at ambient conditions using standard wet chemistry
techniques.
[0028] As will be described below, it has been found that there is
a correlation between the absorbance of the asphaltenes
precipitated using higher boiling point, higher order alkanes
(e.g., higher order than heptane (C7), such as C10, C12, and C14)
and the content of asphaltenes determined at ambient conditions
using conventional wet chemistry techniques using heptane (C7),
such as ASTM D6560. Therefore, the content of asphaltenes in an oil
sample under elevated temperature conditions can be determined
based, in part, on (1) the optical spectrum of the asphaltenes
obtained by precipitation with a first alkane precipitant (e.g.,
higher order alkane) and (2) wet chemistry techniques in a lab
using a second alkane precipitant (e.g., lower order alkane such as
heptane) for the same oil under consideration.
[0029] Moreover, as will be described in further detail below, it
has been found that absorbance data of asphaltenes precipitated
from an oil sample using the above-mentioned first (e.g., higher
order) alkane precipitant can be correlated with absorbance data of
asphaltenes precipitated from the oil sample using a third alkane
precipitant (e.g., another higher order alkane different from the
first alkane precipitant). Therefore, after the asphaltene content
of oil precipitated with the above-mentioned first precipitant has
been determined, as discussed above, (i.e., as a function of the
first precipitant used to obtain the absorbance data and the
measured absorbance of the asphaltenes), the asphaltene content of
the same oil precipitated using a third alkane precipitant (e.g.,
C12) can also be determined by correlating the absorbance data of
asphaltenes precipitated using the third precipitant to the
absorbance data of asphaltenes precipitated using the first
precipitant.
[0030] It will be appreciated that the foregoing discussion may be
clarified by referring to the following details related to the
concepts discussed above.
[0031] FIG. 1B provides a flow chart representing an illustrative
embodiment of a method for determining the asphaltene content of an
oil at elevated temperature conditions. In the illustrated
embodiment, an oil sample is obtained (block 101). It should be
noted that the obtained sample may be a sample that is retrieved
and transported to another location, such as a laboratory, for
analysis, or a sample that is retrieved and analyzed in the field,
as is discussed in greater detail herein. The system and method of
the present invention are also capable of being installed and used
in a downhole tool. The scope of the present invention is not
limited by the means by which the oil sample is obtained.
[0032] The optical spectrum of the oil sample, which may be diluted
with a solvent such as toluene, xylene, or 1-methylnaphthalene at
block 102, is measured at block 103. An undiluted portion of the
oil sample is then mixed with a first precipitant at block 105 to
precipitate the asphaltenes from the oil. In one embodiment, the
first precipitant is an alkane of higher order than heptane (C7),
such as tetradecane (C14). The precipitant may be mixed at a ratio
of one part oil to 40 parts tetradecane or other suitable ratio.
However, other precipitants, such as decane, dodecane or the like,
and other mixing ratios are contemplated by the present
disclosure.
[0033] After the precipitant mixes with the oil, the asphaltenes
will precipitate out of solution so that the precipitated
asphaltenes may be filtered in order to remove the precipitated
asphaltenes from the oil-precipitant mixture (block 107). The
portion of the oil remaining after the precipitated asphaltenes are
removed comprises maltenes, which are species having lower
molecular weights than asphaltenes and are soluble in the first
precipitant. The optical spectrum of the maltenes is measured
(block 109), which is then subtracted from the optical spectrum of
the oil prior to the asphaltenes being removed (block 111). The
resulting optical spectrum is the optical spectrum of the
asphaltenes in the original oil sample. A predicted value of the
weight percentage of asphaltenes in the oil can be determined as a
function of the first precipitant and the asphaltene optical
spectrum (block 113).
[0034] FIG. 2 shows asphaltene absorbance as a function of the
precipitant for five different crude oil samples. Each sample has a
different content of asphaltenes. It is known that higher
absorbance corresponds to higher content of precipitated
asphaltenes. Thus, as shown in FIG. 2, for each of the five
samples, the absorbance (and the amount of precipitated
asphaltenes), decreases as the carbon chain length increases (i.e.,
as the order of alkanes increases, e.g., from hexane (C6) to
tetradecane (C14)).
[0035] Also notable from the plot in FIG. 2 is that the change in
absorbance is proportional to concentration. For example, for crude
oil samples having higher asphaltene content (i.e., higher
absorbance), such as samples 4 and 5, there is a greater change in
absorbance from one precipitant to another as compared to samples
having lower asphaltene content, such as samples 2 and 3.
Nevertheless, even for both crude oil samples having high and low
asphaltene concentrations, as noted above, the absorbance (the
amount of precipitated asphaltenes) reaches a relative plateau for
carbon chains longer than decane (C10), indicating that such higher
order alkane precipitants may be used for precipitation of
asphaltenes.
[0036] FIG. 3A shows a correlation between absorbance of the
asphaltenes for different alkane precipitants ranging in order from
C6 (hexane) to C14 (tetradecane) versus the weight percent of
asphaltenes precipitated using C7 (heptane). The horizontal axis
shows the weight percentage of asphaltenes in each of the five
crude oil samples precipitated using heptane (C7) at ambient
conditions utilizing standard wet chemistry (e.g., gravimetric
analysis) techniques in a lab. The vertical axis shows the
absorbance data from FIG. 2 plotted for each of the five crude oil
samples and for each precipitant noted in FIG. 2.
[0037] The plot in FIG. 3A indicates that the absorbance of
precipitated asphaltenes precipitated using, say tetradecane (C14),
can be correlated to the content of asphaltenes precipitated using
heptane (C7) using standard wet chemistry techniques. Specifically,
FIG. 3A shows that the content of asphaltenes in a crude oil sample
can be represented as a linear function of the absorbance of the
asphaltenes and the precipitant used to obtain the asphaltene
optical spectrum. For example, the line shown plotted for dodecane
(C12) as the precipitant may be used to correlate absorbance data
for another oil sample back to the mass of asphaltenes obtained
using heptane (C7) precipitated using wet chemistry techniques.
This finding is notable because it may be used to predict the
weight percentage of asphaltenes in oil samples precipitated using
a first precipitant, for example of higher order than heptane (and
also higher boiling point than heptane), by correlating the
respective absorbance data of the asphaltenes back to mass
percentage data of the oil sample precipitated using a second
precipitant, for example of lower order than the first precipitant
(e.g., heptane).
[0038] The plot in FIG. 3B also indicates that the slopes of the
correlation lines change from a maximum of 0.1952 for hexane (C6)
as the precipitant to a minimum of 0.1594 for dodecane (C12) as the
precipitant. Also, the difference between the slopes for using
dodecane (C12) and tetradecane (C14) as precipitants is considered
to be within the resolution of the method, further indicating that
use of higher order alkanes beyond decane (C10), does not
appreciably alter the results of the analysis of the asphaltene
concentration.
[0039] Moreover, it has been found that the absorbance of
asphaltenes precipitated using different alkane precipitants
correlate to each other as a function of molecular size of the
precipitant. Therefore, by measuring the absorbance of asphaltenes
using a first precipitant, the absorbance of asphaltenes
precipitated using a second precipitant can be estimated using a
linear correlation.
[0040] FIG. 4A shows the absorbance measurements of asphaltenes
precipitated using decane (C10) and tetradecane (C14). The linear
correlation shows a fit with an R-squared of 0.9949, which
indicates that the absorbances of asphaltenes precipitated using at
least those two different alkanes as precipitants are highly
correlated. Also, FIG. 4B shows the absorbance measurements of
asphaltenes precipitated using octane (C8) and tetradecane (C14).
The linear correlation shows a fit with an R-squared of 0.9977,
which indicates that the absorbances of asphaltenes precipitated
using at least these two different alkanes as precipitants are
highly correlated. Therefore, it is possible to measure the
absorbance of asphaltenes that are precipitated using tetradecane
(C14) and correlate the absorbance back to asphaltenes that are
precipitated using another precipitant, such as decane (C10). Then,
if the asphaltene optical spectrum absorbance data obtained from
precipitating oil with decane (C10) has already been correlated to
the content of asphaltene using wet chemistry techniques, as
described above, it is also possible to correlate the absorbance
data obtained with tetradecane (C14) to the content of asphaltene.
It is also possible to directly correlate the optical absorbance of
asphaltenes obtained using C14 to the weight percentage using
C7.
[0041] FIG. 5 depicts an illustrative embodiment of an apparatus
501 for automated fluid analysis of an oil sample using the
techniques described with reference to the workflow of FIG. 1B. The
apparatus 501 characterizes the asphaltene fraction using
spectroscopy. The apparatus includes a reservoir 503 that holds an
oil sample. The oil sample can include lighter (more volatile)
molecular weight oil components as well as heavy (less volatile)
molecular weight components such as heavy oil and bitumen. An
injection/metering device 505 can be operated to inject a defined
volumetric slug of the oil sample held by the reservoir 503 into a
mixer 507.
[0042] The apparatus 501 also includes a solvent reservoir 517 from
which a solvent such as toluene, xylene, or 1-methylnaphthalene may
optionally be injected to dilute and thereby lighten the oil
sample.
[0043] Finally, the apparatus 501 includes a reservoir 515 that
holds a precipitant that causes asphaltenes to precipitate from an
oil sample when present. The precipitant is injected into the mixer
507 using the injection/metering device 505.
[0044] The mixer 507 can employ chaotic split and recombine
microfluidic mixing techniques or other suitable microfluidic
techniques as described in Nguyen and Wu, "Micromixers--a Review,"
Journal of Micromechanics and Microengineering 15, no. 2 (2005):
R1, herein incorporated by reference in its entirety.
[0045] The injection/metering device 505 can be operated to inject
the precipitant alone, the oil alone, or a mixture of a controlled
ratio of the precipitant and the oil into the mixer 507. The
injection/metering device 505 can be electrically-controlled
syringe pumps, such as the Mitos Duo XS-Pump available from The
Dolomite Center Limited of Royston, United Kingdom, where the
syringe of the respective syringe pumps acts as the reservoirs 503
and 515 that hold an amount of the oil and precipitant,
respectively.
[0046] When the oil and the precipitant mix in the mixer 507 solid
asphaltene content 529 (commonly referred to as asphaltene
floccules or asphaltene flock) precipitate from the mixture 527.
The asphaltene flock 529 is carried as a suspension in the liquid
phase content of the mixture 527. The liquid phase content of the
mixture 527 includes maltenes of the oil sample, which are the
lower molecular weight components of the oil sample that remain
after removing the precipitated asphaltene content. The maltenes
are also soluble in the precipitant.
[0047] The mixer 507 is fluidly coupled to a filter 519 that
provides filtering to trap solid phase hydrocarbon components
(i.e., the asphaltene flock 529), while passing soluble liquid
phase hydrocarbon components (the permeate 530, which includes the
maltenes of the oil sample) to a flow-through optical flow cell
605. The filter 519 may be microfluidic and can also be fluidly
coupled to a waste port that allows for flushing and removal of the
solid phase hydrocarbon components (i.e., the asphaltene flock 529)
that is trapped by the filter 519.
[0048] A spectrometer 607 is optically coupled to the flow-through
optical flow cell 605 and can be operated to derive an optical
spectrum 531 of the fluid that flows from the filter 519 through
the optical flow cell 605.
[0049] In one embodiment, the optical flow cell 605 can be realized
by an optical absorbance flow cell, such as the FIAlab SMA-Z-2.5
cell will fused silica windows and a 2.5 mm optical path and a 2.0
.mu.l internal volume available from FIAlab Instruments, Inc. of
Bellevue, Wash., USA. The spectrometer 607 can be realized by a
broadband spectrometer, such as the model HR2000+ available from
Ocean Optics, Inc. of Dunedin, Fla., USA. The broadband
spectrometer can be used in conjunction with a broadband light
source which can be based on a tungsten filament bulb (such as the
model LS-1 light source available from Ocean Optics, Inc.). Fiber
optic waveguides can be used to optically couple the optical flow
cell 605 to both a broadband light source and spectrometer 607.
[0050] A computer processing system 523 can be programmed with
suitable control logic that interfaces to the injection/metering
device 505 via wired or wireless signal paths therebetween. The
computer processing system 523 can also interface to the
spectrometer 607 via wired or wireless signal paths therebetween.
The control logic of the computer processing system 523 (which can
be embodied in software that is loaded from persistent memory and
executed in the computing platform of the computer processing
system 523) is configured to control the different parts of the
apparatus 501 to carry out an automated sequence of operations
(workflow) that characterizes the asphaltene fraction of an oil
sample. The control logic can be configured by a testing script,
which is input into and executed by the computer processing system
523 to perform automatic control operations as specified by the
testing script. The computer processing system 523 can include a
graphical user interface 533 that allows the user to specify the
sequence of automatic control operations and/or the parameters
(such as pressures, flow rates, temperatures, etc.) for such
automatic control operations. An example of such an automated
workflow is shown in FIG. 1B.
[0051] The workflow shown in FIG. 1B can be performed to measure
the weight concentration of asphaltenes in a sample of oil.
[0052] At block 101, a sample of oil is introduced to the mixer
507, by injection/metering device 505. The oil is then passed
through the filter 519 and to the optical flow cell 605.
[0053] At block 102 the oil sample may optionally be diluted in
mixer 507 with a solvent such as toluene, xylene, or
1-methylnaphthalene before moving to the optical flow cell 605.
[0054] At block 103 the spectrometer 607 analyzes the oil in the
optical flow cell 605 and determines an optical spectrum of the
oil, represented by graph 531. In the illustrated embodiment, the
optical spectrum of the oil, i.e., represented by graph 531, is fed
to computer processing system 523. The computer processing system
523 is configured to store the optical spectrum as measured in
block 103. The computer processing system 523 can determine the
average oil absorbance (Avg.sub.Oil) from the optical spectrum
data. Alternatively, the spectrometer 607 can be configured to
analyze the oil sample and determine the average oil absorbance,
which can then be stored in the computer processing system 523. The
flow path of oil in the apparatus 501 is then cleaned.
[0055] It is not expected that asphaltenes will be collected by the
filter 519 during the operation of block 103. However, in the event
that asphaltenes are collected by the filter 519 during the
operation of block 103, a cleaning procedure can be executed to
remove the collected asphaltenes before continuing to block 105.
This cleaning procedure can involve flowing solvent first across
the filter 519.
[0056] At block 105 samples of oil and dodecane (i.e., a higher
order alkane precipitant) are transmitted to the mixer 507 by
injection/metering device 505. The flow rates for the oil and the
dodecane are configured such that the mixer 507 forms a mixture
where the oil sample is diluted with a predetermined concentration
of the precipitant. The volume fraction of the precipitant in the
mixture can possibly be at or near 40:1 for many oil samples. The
sample of oil and the precipitant are mixed in the mixer 507 at a
predetermined ratio, such as at a ratio of about one part oil to
about 40 parts precipitant.
[0057] At block 107, once the sample of oil and the precipitant are
mixed, the precipitant causes the asphaltenes in the oil to
aggregate and precipitate out in the mixer 507 and the channel 527.
The mixer 507 and the channel 527 can allow the precipitant of the
sample/precipitant mixture produced by the mixer 507 to precipitate
out most if not all of the asphaltene content of the
sample/precipitant mixture (if any asphaltene content is present
from the oil sample). The resultant sample/precipitant mixture
(including the precipitated solid-form asphaltene content) that is
produced flows downstream to filter 519, which traps the
precipitated solid-form asphaltene content and allows the permeate
(i.e., the liquid phase of the sample/precipitant mixture) to pass
to the optical flow cell 605.
[0058] In block 109, the spectrometer 607 is configured to measure
an optical spectrum (represented by graph 531) of the permeate
(which includes the maltenes of the oil sample) that flows through
the corresponding optical flow cell 605. In this manner, the
spectrometer 607 measures an optical spectrum of the maltenes. The
computer processing system 523 is further configured to store the
optical spectrum of the maltenes as measured in block 109.
[0059] At block 109 the computer processing system 523 can
determine an average maltenes absorbance (Avg.sub.Malt) from the
optical spectrum data. Alternatively, the spectrometer 607 can be
configured to analyze the maltenes and determine the average
maltenes absorbance, which can then be stored in the computer
processing system 523.
[0060] At block 111, the computer processing system 523 processes
the optical spectrum measured and stored in block 103 (with the
asphaltene content present and dissolved in the oil sample or oil
sample/solvent mixture) in conjunction with the optical spectrum
measured and stored in block 109 (with the asphaltene content
precipitated and removed from the oil sample/precipitant mixture)
in order to derive the weight fraction of asphaltene in the oil
sample in block 113. In one example, the processing of block 111
can involve deriving a characteristic optical density or absorbance
AU of the asphaltene content of the oil sample by the following
equation:
AU=(OD@600 nm.sub.Spectrum of 103-OD@800 nm.sub.Spectrum of
103)-(OD@600 nm.sub.Spectrum of 109-OD@800 nm.sub.Spectrum of 109).
(1)
The first term of Eq. (1) is derived from the optical spectrum of
block 103 and represents the contribution of both asphaltene
content and the maltenes to AU. The second term of Eq. (1) is
derived from the optical spectrum of block 109 and represents the
contribution of the maltenes alone to AU. The subtraction of the
optical density (OD) at 800 nm in both the first and second terms
is meant to reduce the error from spectral offset introduced by
light scattering and from other errors in the measurements. The
characteristic optical density AU of the asphaltene content as
derived from Eq. (1) can be used to determine the asphaltene
content in the oil sample as a function of the precipitant and the
asphaltene optical spectrum, as discussed above.
[0061] At block 113 the characteristic optical density AU of the
asphaltene content as derived from Eq. (1) is used to determine the
asphaltene content in the oil sample as a function of the
precipitant and the asphaltene optical spectrum, as discussed
above.
[0062] FIG. 6 illustrates an exemplary petroleum reservoir analysis
system 1 in which the disclosed apparatus may be embodied. The
system 1 includes a borehole tool 10 suspended in the borehole 12
from the lower end of a typical multiconductor cable 15 that is
spooled in a usual fashion on a suitable winch on the formation
surface. The cable 15 is electrically coupled to an electrical
control system 18 on the formation surface. The tool 10 includes an
elongated body 19, which encloses the downhole portion of the tool
control system 16. The elongated body 19 also carries a selectively
extendable fluid admitting assembly 20 and a selectively extendable
tool anchoring member 21 which are respectively arranged on
opposite sides of the tool body. The fluid admitting assembly 20 is
equipped for selectively sealing off or isolating selected portions
of the wall of the borehole 12 such that pressure or fluid
communication with the adjacent earth formation 14 is established.
Also included with tool 10 are means for determining the downhole
pressure and temperature and a fluid analysis module 25 through
which the obtained fluid flows. The fluid may thereafter be
expelled through a port or it may be sent to one or more fluid
collecting chambers 22 and 23, which may receive and retain the
fluids obtained from the formation. Control of the fluid admitting
assembly 20, the fluid analysis module 25, and the flow path to the
collecting chambers 22, 23 is maintained by the tool and electrical
control systems 16 and 18. As will be appreciated by those skilled
in the art, the surface-located electrical control system 18
includes data processing functionality (e.g., one or more
microprocessors, associated memory, and other hardware and/or
software) to implement the apparatus as described herein. The
electrical control system 18 can also be realized by a distributed
data processing system wherein data measured by the tool 10 is
communicated (preferably in real-time) over a communication link
(typically a satellite link) to a remote location for data analysis
as described herein. The data analysis can be carried out on a
workstation or other suitable data processing system (such as a
computer cluster, computing grid, etc.).
[0063] In accordance with the present disclosure the tool 10 of
FIG. 6 may function as the apparatus 501 of FIG. 5 and in
accordance with the workflow of FIG. 1B to characterize the
compositional components of a reservoir of interest and analyze
fluid properties of the reservoir of interest based upon its
compositional components. For example, one of the fluid collecting
chambers 22 and 23 may correspond to the oil sample reservoir 503
and precipitant reservoir 515, respectively, of the apparatus 501
of FIG. 5. Also, the electrical control system 18 may correspond to
the computer processing system 523 of the apparatus 501 of FIG. 5.
A sample of formation fluid may be obtained at one or more
reference stations within the borehole 12 at the reservoir pressure
and temperature. The obtained formation fluid samples may be stored
in the collecting chambers 22 and 23 and processed by a fluid
analysis module 25, which may include the functionality of the
injection/metering device 505, mixer 507, filter 519, optical flow
cell 605, and spectrometer 607 of the apparatus 501. Thus, in at
least one embodiment, the fluid analysis module 25 may measure
absorption spectra of oil and deasphaltated oil and translate such
measurements into concentrations of asphaltenes in the oil in the
formation in accordance with the workflow of FIG. 1B.
[0064] The particular embodiments disclosed above are illustrative
only, as the disclosed method may be modified and practiced in
different but equivalent manners apparent to those skilled in the
art having the benefit of the teachings herein. Furthermore, no
limitations are intended to the details of construction or design
herein shown, other than as described in the claims below. It is
therefore evident that the particular embodiments disclosed above
may be altered or modified and all such variations are considered
within the scope of the invention. Accordingly, the protection
sought herein is as set forth in the claims below. Although the
present method is shown in a limited number of forms, it is not
limited to just these forms, but is amenable to various changes and
modifications.
* * * * *